1. Field of the invention
The present invention relates to a method for changing a force control gain and a die cushion control apparatus for changing the force control gain expressed as a ratio of force error to a velocity command value in a control circuit which uses a servo motor to perform force control of a die cushion such that a force of a specified value may be produced by a slide in a press in the interaction with the die cushion disposed opposite to the slide.
2. Description of the Related Art
Generally, in a method for controlling a die cushion in force control using a servo motor such that a force of a specified value may be produced between a slide and a die cushion, a force error is multiplied by a force control gain to obtain a velocity command value, and the velocity command value obtained in this manner is used to control driving of the servo motor. The force error is the difference between a preset force command value and an actual detection value of the force produced between the slide and the die cushion.
The force control gain is determined prior to actual operation of a press machine, and during the operation of the press machine, the determined gain is used for force control of a servo motor to carry out the operation of the press machine. If the determined value of the force control gain is not appropriate, a control system may become unstable due to oscillations or excessive overshoots. If the force control gain is too great, the press machine or the accessories of the press machine may be broken or damaged. Therefore, a force control gain is usually determined as an upper limit of a range such that oscillations or overshoots may not exceed a specified value.
In an exemplary method for determining the force control gain, a constant force is applied in the direction of an axis of force control, and while the gain is incrementally increased, starting from a sufficiently low gain, a magnitude of overshoot is detected for each force command value until an upper limit of the force control gain is obtained. The force control gain obtained in this manner needs to be changed as required, since an optimal value changes because of the aging of press machines, change of material of work pieces, or individual variation of press machines.
Although not directly related to the present invention, an exemplary method in related art is disclosed in Japanese Patent Publication No. H10-202327. In this method, a die cushion is controlled by a torque command to a servo motor. Also, a method is disclosed in Japanese Patent Publication No. 2006-130524, in which a die cushion is controlled by velocity command to a servo motor such that a force control is performed with the velocity command corrected in accordance with the detected velocity of a slide.
However, there is a problem with a force control gain that may be affected by the operational environment (such as inertia, mass, rigidity), and high skill is necessary in order to obtain an accurate value. There is also another problem that, if the force control gain is adjusted incrementally, starting from a low initial value of force control gain within a range that does not exceed the upper limit of a specified overshoot, the number of repeated trials become too excessive and a gain adjustment takes too much time.